The invention relates to an apparatus for measuring the concentration of a liquid, and more particularly, to such apparatus which measures the concentration of colored fine grains contained in a liquid developer.
As is well recognized, a liquid developer comprises a dispersion of colored fine grains in a liquid dispersion medium, and is usually used to develop an electrostatic latent image. As the developing process proceeds, the colored fine grain component in the liquid developer is consumed. Alternatively, when the liquid developer is allowed to stand over a prolonged period of time, the dispersion medium becomes vaporized, and the proportion of colored fine grains mixed with the dispersion medium or the concentration thereof may change. Thus, there arises a problem that the developing process is not stabilized. In order to maintain the liquid developer at a constant concentration, the concentration may be determined photoelectrically, and either a concentrating toner or dispersion medium (carrier solution) may be supplied in response to the result of the measurement. In these instances, colored fine grains or toner particles may be deposited on the surface of a concentration measuring cell or a photoelectric sensor, causing an error in the measurement.
The problem may be eliminated by an apparatus for measuring the concentration of a developer, as disclosed in Japanese Laid-Open Patent Application No. 63,243/1981, for example. Such arrangement is illustrated in FIGS. 13A and 13B. As shown, a liquid developer is introduced into a flat nozzle 1 which is disposed in a circulating path therefor. As the developer is projected from the flat end of the nozzle 1, it forms a thin liquid film 2. Accordingly, a light source 3 and a photoelectric cell 4 together with an aligned pair of electrodes 5 and 6 are disposed on opposite sides of the liquid film 2. In this manner, the concentration of the developer is photoelectrically detected by the combination of the source 3 and the cell 4, but because the thickness of the liquid film 2 tends to be unstable, the pair of electrodes 5 and 6 is used to measure the capacitance across the electrodes to calculate the thickness of the liquid film 2 therefrom, thus providing a correction to the result obtained from the cell 4.
An advantage of the described apparatus is the fact that no inconvenience is caused by the deposition of toner particles on a measuring unit to cause an error in the measurement of the concentration. However, since the photoelectric unit is spaced from the film thickness determining unit, there is no assurance that the liquid film 2 will be maintained in the same configuration thereacross, again causing an error. In the cited application, there is disclosed a capacitance technique and an orthogonal projection technique to determine the film thickness. However, both techniques are incapable of determining the film thickness with a high degree of precision. In summary, it is difficult to determine the concentration to a high degree of precision with the conventional arrangements.